1 Update on MiniBooNE H. A. Tanaka a a Department of Physics, Joseph Henry Laboratories Princeton University Princeton, New Jersey 08544, USA MiniBooNE (Booster Neutrino Experiment) is searching for ν µ → ν e oscillations in the neutrino beam produced by the 8 GeV Booster synchrotron at Fermilab. The Booster has delivered 3 . 66 × 10 20 protons-on-target with over 380 thousand neutrino recorded in the detector since September 2002. MiniBooNE is now accumulating enough data to achieve its goal of conclusively confirming or refuting the evidence for neutrino oscillations observed by the LSND experiment. trino interactions are dominated by the charged 1. Introduction current quasi-elastic interaction (CCQE), which The MiniBooNE detector is a 610 cm ra- comprise about 40% of the events. Neutral cur- dius sphere filled with mineral oil instrumented rent elastic scattering and resonant single pion with photomultipliers. The detector is divided production (both neutral and charged current) into two optically isolated concentric regions; an comprise nearly the rest. outer veto region with 240 photomultipliers and an inner “tank” region with 1280 photomultipli- ers. Neutrino interactions are detected via the 2. Physics Cherenkov radiation and scintillation light pro- The primary physics goal of MiniBooNE is to duced by charged particles passing through the confirm the evidence for ν µ → ν e oscillations ob- mineral oil. The veto detects charged particles served by the LSND experiment [1]. The evidence entering or exiting the tank region and is used suggests a ∆ m 2 ranging from 10 − 1 − 10 1 eV 2 , with to reject cosmic muons and select contained neu- ∼ 0 . 25% oscillation probability. The 540 meter trino interactions. distance of the detector from the target is chosen The neutrino beam is produced by protons to reproduce the L/E distribution of the ν e excess from the 8 GeV Booster synchrotron at FNAL. in the LSND experiment ( ∼ 1m / MeV) and max- At design intensity, 5 × 10 12 protons are extracted imize the sensitivity of the experiment to these to the MiniBooNE beamline in a 1.6 µ sec pulse oscillations. at a rate of 5 Hz. The beam is incident on a The phenomena of neutrino oscillations, consid- beryllium target inserted inside a magnetic horn, ered speculative only a decade ago, are now where secondary pions and kaons are produced definitively established in two modes: the “solar” and focussed into the 50 meter-long decay region. ν e → ν x oscillations with ∆ m 2 ∼ 8 × 10 − 5 eV 2 The subsequent decay of the secondary particles and large (but not maximal) mixing [2][3][4][5] produce a nearly pure ν µ beam, with average en- [6], and the “atmospheric” ν µ → ν x oscillations ergy of 800 MeV and O (10 − 3 ) ν e contamination. with ∆ m 2 ∼ 2 . 5 × 10 − 3 eV 2 and maximal mixing The small ν e content is important for the sensi- [7][8][9]. tive search for ν µ → ν e oscillation that is the goal The evidence for oscillations reported by of the experiment. The expected neutrino energy LSND, however, remains unconfirmed by other distribution at the detector is shown in Figure 1. experiments. Its place in the phenomenology In this energy range, the cross section for neu- of neutrino oscillations is intriguing, since the
µ µ 2 wise, the two-ring topology of these events can Fraction of ν Flux / 0.1 GeV ν Flux be used to reject them. The particle algorithms -1 10 ν e Flux incorporate parameters describing the ring sharp- ness and the overall profile to reject events with -2 muon-like rings and multiple rings. 10 A second class of backgrounds come from interac- tions of ν e intrinsically present in the beam from -3 10 the decay of muons and kaons in the decay re- gion. This background is irreducible; it cannot be distinguished in any way from the signal pro- -4 10 cess apart from its overall energy distribution. The expected signal and background rates, as- suming ∆ m 2 = 1 eV 2 and sin 2 2 θ = 0 . 002 and -5 10 0 0.5 1 1.5 2 2.5 3 10 21 protons-on-target are shown in Table 1 with E ν (GeV) and without selection. The expected sensitivity Figure 1. Energy distribution of neutrinos at using a fit to the energy spectrum of the expected MiniBooNE obtained from Monte Carlo simula- events is shown in Figure 2. tion. 4. Systematic Studies In order to estimate reliably both classes of three known active flavors of neutrinos are unable background in the analysis, a detailed under- to simultaneously accommodate the three ∆ m 2 standing of both the neutrino beam and the de- regimes observed by the experiments. A dras- tector performance is needed. A two-prong effort tic remedy to the Standard Model (minimally ex- using offline measurements to complement detec- tended to include neutrino oscillations) is needed tor data is in place to develop this understanding if the phenomenon is confirmed. and reduce the systematic uncertainties. For the misidentified ν µ events, a precise esti- 3. Identifying ν e Events in MiniBooNE mate of the event rates in the detector and an MiniBooNE will search for ν µ → ν e oscillations accurate simulation of the detector response to by detecting an excess of ν e CCQE interactions these interactions is necessary. This allows accu- in the detector. This signal process is identified rate predictions of the performance of the particle by the spatial and temporal distribution of hits identification algorithms used to identify the sig- reconstructed in the photomultiplier array. For nal process. The experiment is currently under- example, the muons emerging from ν µ CCQE in- taking a detailed study of the detector behavior teractions produce Cherenkov ring distributions using the detector calibration systems. Neutrino with cleaner edges than the electrons from ν e interactions observed in the detector, including the background ν µ -induced π 0 production, are CCQE interactions, which undergo multiple scat- tering in the medium. also being analyzed [10]. These in situ efforts are The primary backgrounds to the oscillation ν e complemented by ex situ measurements of oil op- CCQE interactions take two forms. The first are tical properties, including scattering, fluorescence misidentified ν µ events, primarily neutral current and scintillation measurements [11]. π 0 events. The photons emerging from the decay MiniBooNE collaborators are active in the of the π 0 convert in the medium, producing an HARP experiment, where the kaon production e + e − pair. If the photons are highly asymmetric rates needed to estimate the intrinsic ν e back- in energy or have small opening angle, the pho- ground are being measured [12]. The latter mea- tons will appear much like the primary electron surements are cross-checked by the Little Muon emerging from a ν e CCQE interaction. Other- Counter (LMC), a spectrometer that measures
3 Process All Events After Selection ν µ CCQE 553 × 10 3 8 ν µ NC π 0 110 × 10 3 290 1 × 10 3 ν µ ∆ → ( n/p ) γ 80 2 . 5 × 10 3 Intrinsic ν e 350 1 . 5 × 10 3 Oscillation signal 300 Table 1 The expected event yields for the primary back- ground channels and the oscillation signal with ∆ m 2 = 1 eV 2 and sin 2 2 θ = 0 . 002 and 10 21 protons-on-target. the spectrum and rate of wide-angle muon pro- duction resulting from kaon decay in the 50 meter decay region[13]. Figure 2. The expected sensitivity to ν µ → ν e 5. Current Status and Outlook oscillations at MiniBooNE with 10 21 protons-on- The MiniBooNE detector has recorded nearly target delivered to the target. four hundred thousand neutrino interactions since data-taking began in 2002. During this time, the Fermilab Booster has delivered 3 . 66 × 10 20 pro- 2. Y. Fukuda et al. , Phys. Rev. Lett. 77 :1683- tons to the beryllium target used to produce the 1686, 1996 (Kamiokande). neutrino beam. The experiment is now accumu- 3. J. N. Abdurashitov al. , Phys. Rev. et lating the 10 21 protons-on-target needed to make Lett. 83 :4686-4689, 1999 (SAGE). a conclusive confirmation or refutation of the 4. M. Altmann et al. , Phys. Lett. B490 , 16, LSND evidence for neutrino oscillations, while 1999 (GALLEX). completing systematic studies necessary for reli- 5. S. Ahmed et al. , Phys. Rev. Lett. 92 :181301, ably estimating signal efficiencies and background 2004 (SNO). rates. 6. K. Eguchi et al. , Phys. Rev .Lett. 92 :071301, 2004 (KamLAND). 6. Acknowledgments 7. S. Fukuda et al. , Phys. Rev .Lett. 86 :5656- 5660, 2001 (Super-Kamiokande). The collaborating institutions thank the Fermi 8. S. Fukuda et al. , Phys. Rev. Lett. 85 :3999- National Accelerator Laboratory (FNAL) for its 4003, 2000 (Super-Kamiokande). kind hospitality. We are grateful for the excel- 9. M.H. Ahn et al. , Phys. Rev. Lett. 90 :041801, lent performance provided by the Accelerator Di- 2001. (K2K). vision and the strong support of the Directorate 10. H.A. Tanaka, these proceedings. of FNAL. This work is funded by the Department 11. M. O. Wascko, DPF 2004, Riverside, Califor- of Energy and the National Science Foundation of nia, USA, 2004. the United States. 12. I. Kato, these proceedings. 13. E. D. Zimmerman, 4th International Work- REFERENCES shop on the Neutrino Beams and Instrumen- tation, Tsukuba, Japan, 2003. 1. A. Aguilar et al. , Phys.Rev. D64 :112007, 2001 (LSND).
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